Microwave signal delay apparatus

ABSTRACT

There is disclosed a microwave signal delay device employing an M-type traveling wave tube. The drift space of this tube is of circular shape with a mean radius of Rm, and the drift electrodes are similarly curved with a radius of curvature equal to Rm/2. With this relationship, all of the electrons of the beam have substantially the same angular velocity and travel around the drift space in equal times. This minimizes any velocity slip in the beam.

United States Patent Belohoubek Feb. 1,1972

[54] MICROWAVE SIGNAL DELAY APPARATUS [72] inventor: Erwin F. Belohoubek, Kendall Park, NJ.

[73] Assignee: The United States of A ica as represented by the Secretary of Navy [22] Filed: Aug. 12, 1966 [21] Appl.No.: 572,153

[56] References Cited UNITED STATES PATENTS 2,801,362 7/1957 Hebenstreit et a1 ..3l5/3 A 2,812, 11/1957 Mourier 15/393 3,102, 8/1963 Sturrock /39.3 X 3,153,742 10/1964 Kliiver ..315/39.3 X 3,270,244 8/1966 Ayaki ..3 15/39 Primary ExaminerRodney D. Bennett, Jr.

Assistant Examiner-H. A. B' 'el AttorneyR. I. Tompkins an 1. Shrago [57] ABSTRACT There is disclosed a microwave a1 delay device employing an M-type traveling wave tube. e drift space of this tube is of circular shape with a mea dius of R,,,, and the drift electrodes are similarly curved w a radius of curvatur ual to R /2. With this relationship, all of the electrons of beam have substantially the same angular velocity and travel around the drift space in equal times. This minimizes any velocity slip in the beam.

6 Claims, 2 Drawing Figures MICROWAVE SIGNAL DELAY APPARATUS The present invention relates generally to signal delay devices and, more particularly, to an adjustable microwave signal delay tube employing an electron beam traveling through crossed electric and magnetic fields. f Traveling wave tubes have been used in the past as variable delay lines. In one type of apparatus, the so-called M-type," the electron beam moves through crossed electric and magnetic fields. The signal which is to be delayed is transferred to the beam at an input coupler and, thereafter, the modulated beam proceeds'at an adjustable velocity through a drift section After the beam traverses this section, it'passes through an output coupler, which extracts the signal information, and

then it terminates at a collector electrode.

jlhe total signaldelay of such an M-type traveling wave tube as the one just described corresponds to the time it takes for.

tainable is the signal loss brought about by velocity slip between different sections of the electron beam. This slip, which results in a loss in the microwave signal and a deteriora t ion in its waveform, is caused by the space-charge potential depression in the beam, mechanical differences in the spacingofthe drift plates, and variations in the magnetic field. ln'all of these instances, either the electric or magnetic field in one section of the beam is slightly different from that in another. Therefore, difi'erent electron velocities occur within the beam.

One of the major requirements for the satisfactory operation.'of an M-type delay tube is the proper focusing of the beam at the low-drift velocity. The end-hats commonly used for the side focusing of the beam produce fringing fields, and this distortion causes velocity slip at the beam edges. More particularly, these end-hats, which may be flanges having oppositely slanted surfaces, deform the electric field in the drift region sothat there is an inwardly directed or E, component .Of-"Ihfi field that prevents spreading of the beam in the direction parallel to the magnetic field. However, there is an accompanying distortion in the transverse or E, component of the electric field and, thus, the electrons in the outer limits of the beam move at higher velocities than those in the center of the beam. This focusing arrangement, consequently, produces a certain amount of velocity slip.

t The present invention eliminates to a substantial extent this type of slip by curvingthe drift electrodes. By having both drift plates similarly curved, the transverse or E, component of the electric field remains substantially constant over the width of the beam, provided this width is small compared to the radius of curvature of the plates, while the E component of the field, the one relied upon to do the side focusing, is zero at the center of the beam and increases linearly towards the beam edges. Hence, the proper electrical restoring force is present.

Since the close curvature, unfortunately, does introduce a variation in the E, component of the electric field, a certain amountof velocity slip is introduced perpendicular to the direction of the magnetic field. This variation, which is proportional to HR, where R is the radius of curvature of the drift electrodes, is compensated for by curving the drift space of the delay tube in an opposite direction. if the mean radius of curvature of the drift space R,, is equal to R, a saddle point is created at the beam center around which the electrical field component E, perpendicular to the magnetic field is constant to a first order over the whole cross section of the beam. However, for slip-free flow in the curved drift space, the velocity of the electrons must increase proportionally to the radius R to assure constant angular velocity, and thus provide equal transit times for all electrons. If the potential depression by equal to 2R.

In the operation of the usual M-type delay tube, it is neces sary to:change the beam velocity in the drift region by aratio of approximately l00-to-l to achieve an equivalent delay change..Adiabatic transitions in the beam velocity, therefore, have to be provided between the input and output coupler and the drift section. Moreover, an additional transition is required between the electron gun and the input coupler if the beam is to be launched at a velocity sufficiently low to minimize beam spread.

In previous tubes, these transitions were achieved by geometric tapers at these locations. However, this required complicated and time-consuming machining and grinding operations. According to one feature of the present invention, these difficulties are avoided by utilizing a transition wherein the velocity change is accomplished by a voltage drop across a resistive film deposited on the insulating layer of inner and outer tube rings. The resistive coating makes contact on one side with the metal film of the coupler which is kept at a high potential and on the other side with the metal film of the drift section ,whose potential is varied accordingly to the desired delay. Because the drift velocity is determined by the ratio of the electric to magnetic field E,,/B the electrons gradually change-theirvelocity as the voltage difference between the resistive coatings of inner and outer ring varies along the transition. Ifthe resistivity of these films is uniform, a linear varia tion in the transverse electric field results, and'this produces an adiabatic. velocity change over the shortest possible distance. One further advantage of. this type of transition is that it-eliminates the velocity drop which normally occurs. in the center of a geometrically tapered transition and thus permits a lower minimum delay time for the overall apparatus.

It is a primary object of the present invention to provide an improved, adjustable, microwave delay tube of the M-type wherein the signal loss due to the velocity slip in the beam is minimized. J I

A secondary object of the present invention is to provide a traveling wave delay device wherein the focusing arrangement for preventing beam spread does not produce velocity slip.

Another object of the present invention is to provide a microwave delay tube of the M-type wherein the drift electrodes and the drift space are oppositely curved to minimize any slip in the electron beam.

A still further object of the present invention is to provide a crossfield microwave delay device wherein the adiabatic transitions are produced by" resistive coatings applied to the boundary surface of the drift electrodes. I

A yetjstill further objectof the present invention is to provide anfM-type microwave delay tube having'simplified and improved fast wave input and output couplers.

A yet still furtherobject of the present invention is to provide a'microwave signal delay device in the traveling wave tube configuration wherein mechanical variations in the drift spaceare minimized to insure proper electric field intensities.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. its a simplified schematic diagram of one embodiment of the present invention; and

F1652 is a cross-sectional view of the apparatus of FIG. 1 taken through line 2-2, illustrating the curved drift electrodes'and the oppositely curved drift space.

Referring now to FIG. 1 of the drawings, it will be seen that the delay tube in one preferred embodiment comprises an inner ring 1 and a concentrically spaced outer ring 2, both fabricated on nonmagnetic material. to avoid magnetic field distortions in the circular drift space 3 separating them. Inner ring 1 is the primary structural member of the apparatus, and the electron gun 4 which forms and launches the strip-type electron beam, input coupler 5 which transfers the microwave signal to this beam, and output coupler 6 which subsequently extracts this signal after it has been delayed by its passage aroundthe arcuated drift space are all secured to it at appropriately spaced locations.

As perhaps best seen in FIG. 2, which is a cross-sectional view taken along a diameter of the tube, the confronting surfaces of rings 1 and 2 are similarly curved over their complete circumferential lengths. These surfaces have electrically insulating coatings 7 applied thereto. Metallic films are evaporated on these coatings to form the drift electrodes 8 and 9. Both drift electrodes, therefore, have the same curvature R. Because of this, as mentioned hereinbefore, the electric field developed in the drift space in response to an applied voltage across these electrodes has a component E along the width of the strip beam which is zero at the center of this beam and increases linearly in magnitude with distance away from this point. This E, component acts as an inwardly directed restoring force to restrain any electrons near the edges of the beam from further spreading out towards the sidewalls of the drift region. The drift electrodes of the present tube, therefore, additionally serve as a side focusing means for maintaining proper beam width, replacing, for example, the end-hats used in prior art devices.

If the radius of curvature R is large compared to the width of the strip beam, then the transverse or E, component of the electric field is substantially constant over the beam width. However, there is a variation in this transverse component which is proportional to UK. As mentioned in the introduction, the present invention compensates for this variation by utilizing concentric rings 1 and 2 which form an arcuated drift space which has a curvature opposite to that of the drift electrodes. The mean radius of curvature of this drift space R is purposely made equal to 2R. If this relationship is followed, the angular velocity of the electrons is independent of the radius to a first order of magnitude, and minimum velocity slip occurs.

The external magnetic field, which may be produced by conventional means, it will be appreciated, acts at right angles to the electric field throughout the complete drift region and, in FIG. 1, the direction of this field is into the paper as shown by the inscribed cross.

The layer construction of insulation 7 and metallic film 8 applied to rings 1 and 2, it should be appreciated, permits the achievement of a very high accuracy in the spacing between the drift electrodes. Thus, slip losses in the present tube due to structural irregularities are minimized. Another advantage of this layer structure is that it reduces any fast wave propagation between stray fields of the input and output couplers. ln common M-type devices, fast wave propagation generally occurs along the deisolated sole or anode with the vacuum envelope acting as the outer conductor of the transmission line. The insulating layer construction used in the delay tube leads to a very low characteristic impedance for the parallel-plate transmission lines formed by the metal films and the ring members. The low impedance in turn prevents effective coupling from coupler stray fields to these transmission lines and thus eliminates the need for separate damping elements. The decoupling between input and output coupler is also aided by the fact that the metal films are interrupted by the resistive coatings in the transition regions.

Referring back to FIG. 1, the cathode element 11 and the beam forming electrode 12 of the electron gun are accommodated within a cutout portion 13 of inner ring 1, and these elements are maintained as a unitary subassembly at the proper height by insulating rods 14 made, for example, of alumina. The electron gun is operated with a low anode voltage which launches the beam at a low velocity and thereby minimizes its guideline spread. Since the delay tube utilizes fast wave couplers, a first adiabatic transition is necessary between gun 4 and the entry side of input coupler to bring the beam up to proper electron velocity. This transition is achieved by applying a suitable length of resistive film 15, shown as a dotted line, on the inner surface of ring 2 at a location immediately adjacent to the point at which the beam is launched into the annular space separating both rings. This coating may be produced, for example, by the evaporation of carbon on the ring surface, and a portion 16 of this film at a location opposite cathode 11 may be utilized to replace the usual anode structure of the electron gun.

In the adiabatic transitions used in the present delay tube at the three different sites, the velocity change is achieved by the voltage drop across the resistive film. If the resistivity of this film is uniform, a linear variation in the transverse electric field E results, and this produces an adiabatic velocity change over the shortest possible distance. This characteristic is advantageous since it reduces the minimum delay times attainable with the apparatus.

The delay has a second adiabatic transition 22 immediately adjacent the exit side of input coupler 5 which slows the modulated beam down for its passage through drift space 3. There is a third adiabatic transition 23 adjacent the entry side of output coupler 6 which restores the beam to the higher velocity required for the operation of this coupler. In these transitions the resistive films are applied to the inner surfaces of both rings and, again, these films are represented by the dotted lines shown. The inner ring may also be slightly tapered to improve the performance of each transition section at large signal delay conditions.

Each coupler 5 and 6 is a capacitively loaded coaxial resonator with the capacitive area acting as the modulating gap for the beam. The cavity resonator 18 of the input coupler 5 is formed by a straight opening machined directly into inner ring 1. A coaxial line 19 is inserted through an appropriate aperture in the wall of this ring, and its inner conductor 20 terminates at a conducting disk 21 which acts as the above capacitive element. The input signal may be coupled to the inner conductor 20 in any conventional manner. Output coupler 6 is identical to input coupler 5 and, consequently, no description of this unit will be given.

The electronic beam, after it passes through the output coupler, terminates at a collector electrode. The collector electrode of the present invention takes the form of an isolated area of metal film in a drift space of decreasing height.

As perhaps best seen in FIG. 2, a pair of closure plates may be secured by any well-known arrangement to opposite sides of rings 1 and 2 for enclosing and sealing off the interior of the tube. Once these rings are in place and the tube completely assembled, its interior may be evacuated by any well-known technique.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In an M-type traveling wave tube microwave signal delay device, the combination of an inner ring member having an outer surface of conducting material;

an outer ring member having an inner surface of conducting material concentrically spaced therefrom whereby an annular space occurs between said members with the mean radius of curvature of said annular space having a dimension R means for developing across confronting arcuated portions of said inner and outer surfaces of said ring members an electrical field and for developing between these arcuate portions a magnetic field which is at right angles to said electrical field,

said arcuated portions of said inner and outer surfaces corresponding to a pair of boundary surfaces of a drift space;

means for introducing an electron beam into said annular space at a location remote from said drift space;

input means positioned adjacent one end of said drift space for coupling a microwave signal to said electron beam thereby to produce a modulated electron beam;

output means positioned adjacent the other end of said drift space for extracting said microwave signal after said modulated electron beam has traveled through said drift section under the control of said electric and magnetic fields;

means for collecting said electron beam after said microwave signal has been extracted therefrom;

said inner and outer surfaces of said ring members being curved in a direction parallel to said magnetic field with the radius of curvature thereof being equal to the dimen sion R /2, whereby the electrons in said modulated beam all have substantially equal transit times in traveling trough said drift space, thereby to reduce slip in said beam and loss of microwave signal quality.

2. In an arrangement as defined in claim 1 wherein said input means for coupling the microwave signal to said electron beam consists of a first fast wave capacitive coupler cooperating with said annular space through which said electron beam passes prior to its entry into said drift space.

3. In an arrangement as defined in claim 1 wherein said output means for extracting the microwave signal from said modulated beam consists of a second fast wave capacitive coupler cooperating with said annular space through which said modulated electron beam passes after it has traveled through said drift space.

4. In an arrangement as defined in claim 1 wherein an adiabatic transition is provided on opposite sides of said input means for increasing the electron beam velocity prior to its passage through said input means and for reducing the electron beam velocity prior to the modulated electron beams entry into the drift space.

5. In an arrangement as defined in claim 4 wherein each adiabatic transition includes resistive coatings applied to confronting surfaces of said inner and outer ring members.

6. In an arrangement as defined in claim 1 wherein an adiabatic transition is provided for increasing the velocity of the modulated electron beam prior to its passage into said output means which extracts said microwave signal from said modulated electron beam. 

1. In an M-type traveling wave tube microwave signal delay device, the combination of an inner ring member having an outer surface of conducting material; an outer ring member having an inner surface of conducting material concentrically spaced therefrom whereby an annular space occurs between said members with the mean radius of curvature of said annular space having a dimension Rm; means for developing across confronting arcuated portions of said inner and outer surfaces of said ring members an electrical field and for developing between these arcuate portions a magnetic field which is at right angles to said electrical field, said arcuated portions of said inner and outer surfaces corresponding to a pair of boundary surfaces of a drift space; means for introducing an electron beam into said annular space at a location remote from said drift space; input means positioned adjacent one end of said drift space for coupling a microwave signal to said electron beam thereby to produce a modulated electron beam; output means positioned adjacent the other end of said drift space for extracting said microwave signal after said modulated electron beam has traveled through said drift section under the control of said electric and magnetic fields; means for collecting said electron beam after said microwave signal has been extracted therefrom; said inner and outer surfaces of said ring members being curved in a direction parallel to said magnetic field with the radius of curvature thereof being equal to the dimension Rm/2, whereby the electrons in said modulated beam all have substantially equal transit times in traveling trough said drift space, thereby to reduce slip in said beam and loss of microwave signal quality.
 2. In an arrangement as defined in claim 1 wherein said input means for coupling the microwave signal to said electron beam consists of a first fast wave capacitive coupler cooperating with said annular space through which said electron beam passes prior to its entry into said drift space.
 3. In an arrangement as defined in claim 1 wherein said output means for extracting the microwave signal from said modulated beam consists of a second fast wave capacitive coupler cooperating with said annular space through which said modulated electron beam passes after it has traveled through said drift space.
 4. In an arrangement as defined in claim 1 wherein an adiabatic transition is provided on opposite sides of said input means for increasing the electron beam velocity prior to its passage through said input means and for reducing the electron beam velocity prior to the modulated electron beam''s entry into the drift space.
 5. In an arrangement as defined in claim 4 wherein each adiabatic transition includes resistive coatings applied to confronting surfaces of said inner and outer ring members.
 6. In an arrangement aS defined in claim 1 wherein an adiabatic transition is provided for increasing the velocity of the modulated electron beam prior to its passage into said output means which extracts said microwave signal from said modulated electron beam. 